1. Field of the Invention
This invention relates to an improved process for the calcination and activation of a catalyst containing mixed oxides of vanadium and phosphorus (V/P/O) useful in the preparation of maleic anhydride from n-butane. More specifically but not by way of limitation, the present invention relates to a method of calcination of a V/P/O catalyst by inducing a very rapid temperature rise while in an oxidizing fluidized state and a novel method of accomplishing this within a transfer bed commercial scale plant.
2. Description of the Related Art
The vapor phase oxidation of n-butane to maleic anhydride over a V/P/O catalyst is generally known in the art. A review of the prior art describing this process is given in U.S. Pat. No. 4,668,802. In improved processes described in U.S. Pat. No. 4,668,802 and U.S. Pat. No. 5,021,588 the reaction is conducted in a recirculating solids reactor having two fluidized reaction zones. The conversion of n-butane to maleic anhydride takes place primarily in one reaction zone and the catalyst, essentially stripped of gaseous species, is transported to the other reaction zone where it is re-oxidized; i.e., regenerated prior to being returned to the reaction zone.
The mixed oxide V/P/O compositions used as catalysts in these processes are known to the art. A review of the art describing V/P/O compositions and their preparation is given in U.S. Pat. No. 4,371,702. The catalyst precursor composition is usually made by a process wherein a conventional vanadium compound in which the vanadium is in the +5 oxidation state, such as in V.sub.2 O.sub.5 or NH.sub.4 VO.sub.3, is partially reduced to the +4 oxidation state by reaction in either an aqueous or organic liquid medium. The catalyst precursor is then formed by the addition of any appropriate phosphorus compound, for example H.sub.3 PO.sub.4, refluxing to bring about reaction and recovering the catalyst precursor, usually as a hydrated vanadium phosphate, by filtration and drying or spray drying. The prior art describes the desirable range of atomic ratios of phosphorus to vanadium and also the incorporation of catalyst promoters. It is desirable that the V/P/O catalyst has good attrition resistance particularly when it is used in a fluidized bed or recirculating bed reactor. The prior art relevant to attrition resistant catalyst and its preparation is reviewed in U.S. Pat. No. 4,677,084. The production of V/P/O catalysts useful for the preparation of maleic anhydride from n-butane requires controlled calcination and activation of the catalyst precursor. This is accomplished by heating the precursor under appropriate temperature, time and atmosphere conditions to accomplish dehydration, while maintaining the average vanadium oxidation state (Vox) between specified limits. For example in U.S. Pat. No. 3,915,892 the precursor dihydrate is converted to the monohydrate by evolving one water of hydration at 370.degree. C. to 394.degree. C. while maintaining the Vox in the range of 4.1 to 4.5. The balance of the water is then removed by heating at 395.degree. C. to 425.degree. C. followed by promoting a bulk crystal phase transition above 450.degree. C. in a carrier gas consisting of air or an inert gas together with controlled amounts of oxygen and hydrocarbon to provide an effluent stream containing at least one volume percent of oxygen or one volume percent each of oxygen and hydrocarbon, again while maintaining the Vox in the range of 4.1 to 4.5.
When the calcination/activation procedures described in the prior art are used to prepare V/P/O catalyst on the scale necessary to provide catalyst for commercial operation it is found that, from the standpoint of maleic anhydride production from n-butane, the resulting catalyst is inferior, to catalyst made on a small batch basis. Consequently more stringently controlled process conditions are needed to produce high grade catalyst on a scale required for commercial plant operation. For example in U.S. Pat. No. 5,573,992 issued Nov. 12, 1996, a relatively low pressure process for calcination and activation of a catalyst containing mixed oxides of vanadium and phosphorus within limited specified operating conditions is disclosed. This process involved the steps of calcining a hydrated V/P/O or V/P/O-SiO.sub.2 catalyst precursor at a temperature range of 375.degree. C. to 400.degree. C. for sufficient time to dehydrate the catalyst precursor while simultaneously and continuously controlling the oxygen level such as to maintain the Vox in the range of 3.825 to 4.175; and then activating the dehydrated catalyst precursor by further heating at a range of 340.degree. C. to 500.degree. C. in the presence of a gaseous atmosphere containing n-butane at a partial pressure not exceeding 0.20 atm while simultaneously and continuously controlling the oxygen level such as to maintain the Vox in the range of 3.95 to 4.15. The instant invention is viewed as an improvement over this process.